1. Field of the Invention
The present invention relates to an optical transmission apparatus, and in particular to an optical transmission apparatus having a redundant configuration provided on an output side of a submarine optical terminal.
2. Description of the Related Art
FIG. 6 shows a general system configuration of a submarine optical terminal, where an optical multiplexer 1 multiplexing optical signals having wavelengths λ1-λn of n-wave, a preceding stage amplifier 2 amplifying optical signals outputted from the optical multiplexer 1, a dispersion compensating fiber 3 inserted into an optical fiber transmission line so as to cancel a wavelength dispersion on the transmission line for the optical signals amplified by the preceding stage amplifier 2, a subsequent stage amplifier 4 further amplifying optical signals outputted from the dispersion compensating fiber 3, and an optical transmission apparatus 5 transmitting the optical signals amplified by the subsequent stage amplifier 4 to an adjoining relay station are all connected in series. The preceding stage amplifier 2, the subsequent stage amplifier 4, and the optical transmission apparatus 5 are interconnected to a monitoring controller 6.
The optical transmission apparatus 5 in this system configuration is composed of an optical output controller and an optical output portion. The optical output controller has a redundant configuration of a working optical output controller 10 and a protection optical output controller 20, and controls an optical output portion 30 common to both of the controllers. Thus, a redundant configuration of the optical output controller stabilizes the system, and the optical output portion comprising elements whose failure rate is low enhances reliability.
FIG. 7 shows a prior art arrangement of the above-mentioned optical transmission apparatus 5, where this arrangement shows only one of the optical output controllers 10 and 20 as representatively indicated by a reference numeral “0” and the optical output portion 30, for the sake of simplifying the description.
Firstly, the optical transmission apparatus 5 performs a (negative) feedback control including the optical output portion 30 in order that the optical output controller 0 keeps its optical output level fixed and stable.
The optical output controller 0 is composed of an input portion 1 for inputting an optical output monitoring signal Vmon fed back from the optical output portion 30 and for taking out only the DC component thereof, a preset value controller 2 for performing a D/A conversion and an inversion to an output preset value Vref provided from the monitoring controller 6 and for outputting an inverted preset value signal −Vref, a monitoring portion 3 for performing a monitor value/state notification to the monitoring controller 6, an integrating circuit 4 connected to the input portion 1, the preset value controller 2, and the monitoring portion 3, and an output resistor R3 serially connected to the output side of the integrating circuit 4.
Also, the integrating circuit 4 is composed of an operational amplifier OP, input resistors R0 and R1 one ends of which are commonly connected to an inverting input terminal of the operational amplifier OP and the other ends of which are respectively connected to the input portion 1 and the preset value controller 2, and a resistor R2 and a capacitor C1 serially connected between the inverting input terminal and the output terminal of the operational amplifier OP and composing a feedback circuit. A non-inverting input terminal of the operational amplifier OP is grounded through a resistor. It is to be noted that the monitoring portion 3 is connected to the output terminals of the input portion 1 and the preset value controller 2 so that the output signals may be transmitted to the monitoring controller 6 to be monitored.
The optical output portion 30 is composed of a diode D0 connected to the output resistor R3 of the optical output controller 0 and receiving an optical control signal Vcnt, an optical variable attenuator OAT connected to the diode D0, an optical output monitor OM2 monitoring an optical input from the subsequent stage amplifier 4 shown in FIG. 6 and providing the optical input to the optical variable attenuator OAT, and an optical output monitor OM1 outputting optical signals outputted from the optical variable attenuator OAT to a relay station and generating the optical output monitoring signal Vmon. The input terminal of the optical variable attenuator OAT is grounded through a coupling capacitor C100.
FIGS. 8A and 8B show operation waveforms upon output setting of the optical output controller 0 in the optical transmission apparatus 5. It is to be noted that +V and −V indicate a power supply voltage of the operational amplifier OP.
Firstly, the operation principle of the integrating circuit 4 including the operational amplifier OP will now be described. As shown in FIG. 2A, a transfer function G of the integrating circuit in this case can be expressed by the following Eq. (1):
                    G        =                              Vout            Vin                    =                                                    -                                  (                                                            R                      ⁢                                                                                          ⁢                      2                                        +                                          1                                              jω                        ⁢                                                                                                  ⁢                        C                                                                              )                                                            R                ⁢                                                                  ⁢                1                                      =                                                                                -                    ω                                    ⁢                                                                          ⁢                  CR                  ⁢                                                                          ⁢                  2                                +                j                                            ω                ⁢                                                                  ⁢                CR                ⁢                                                                  ⁢                1                                                                        Eq        .                                  ⁢                  (          1          )                    
Accordingly, the gain can be provided by the following Eq. (2):
                                        G                          =                                            1              +                                                (                                      ω                    ⁢                                                                                  ⁢                    CR                    ⁢                                                                                  ⁢                    2                                    )                                2                                                          ω            ⁢                                                  ⁢            CR            ⁢                                                  ⁢            1                                              Eq        .                                  ⁢                  (          2          )                    
Thus, the AC gain can be expressed by the following Eq. (3):
                                        G                          =                              R            ⁢                                                  ⁢            2                                R            ⁢                                                  ⁢            1                                              Eq        .                                  ⁢                  (          3          )                    
When the integrating circuit has two input resistors R0 and R1 as shown in FIG. 7, the output voltage Vout of the operational amplifier OP can be provided by the following Eq. (4):
                    Vout        =                                                            R                ⁢                                                                  ⁢                2                                            R                ⁢                                                                  ⁢                0                                      ⁢                          (              Vmon              )                                +                                                    R                ⁢                                                                  ⁢                2                                            R                ⁢                                                                  ⁢                1                                      ⁢                          (              Vref              )                                                          Eq        .                                  ⁢                  (          4          )                    
If R0, R1, and R2 are equal to each other (R0=R1=R2) in the above-mentioned Eq. (4), the output voltage Vout of the operational amplifier OP assumes Vmon+Vref.
Since the output voltage Vref from the monitoring controller 6 is provided to the resistor R1 as a signal−Vref D/A converted and inverted by the preset value controller 2, a negative feedback control is performed so as to assume Vout=−Vref+Vmon. Namely, when the optical output portion 30 is controlled to the output preset value Vref, a control is performed to the optical output portion 30 so that the output voltage Vout of the operational amplifier OP may assume Vmon=Vref.
This will be specifically described referring to operation waveforms of FIGS. 8A and 8B. Before a start time t1 of the output setting or control from the monitoring controller 6, the output −Vref of the preset value controller 2 assumes “0” as shown in FIG. 8A. It is to be noted that in FIGS. 8A and 8B, the polarity of −Vref is shown in inverted form for easy contrast to Vmon.
Since the optical output of the subsequent stage amplifier 4 is already set at this time, the optical input to the optical output monitor OM2 in the optical output portion 30 exists, and since a faint signal exists at the output of the optical variable attenuator OAT, the optical output monitoring signal assumes Vmon>0. Thus, −Vref(=0)+Vmon>0 is obtained, the output Vout of the operational amplifier OP before the output control (setting) start from the monitoring controller 6 is amplified up to the negative power supply voltage −V and stabilized as shown in FIG. 8B.
When the output control (setting) is started from the monitoring controller 6 at a time of t1 thereafter, the preset value controller 2 outputs the inverted preset value signal −Vref, and the operational amplifier OP controls the output Vout so as to obtain −Vref+Vmon=0, as shown in FIG. 8A. When −Vref+Vmon=0 is obtained, the output Vout can be obtained in a stable form. At this time, the optical control level Vcnt assumes the level of the output preset value Vref However, since the diode D0 is provided on the output side of the operational amplifier OP, a stable output is to be generated in the form of being dropped by the voltage drop from the output voltage Vout of the optical output controller 0.
While the above-mentioned optical transmission apparatus 5 in FIG. 7 has been shown as the series circuit of the single optical output controller 0, and the optical output portion 30, the optical transmission apparatus in FIG. 9 corresponds to the actual redundant configuration shown in FIG. 6. Namely, the output terminals of the two optical output controllers 10 and 20 are interconnected through the wired-OR circuit OR in the optical output portion 30 and connected to the optical variable attenuator OAT.
It is to be noted that the configurations of the optical output controllers 10 and 20 are the same as those of the optical output controller 0 shown in FIG. 7. However, in order to mutually distinguish the optical output controller 10 and the optical output controller 20, the input portion is shown as e.g. an input portion 11 in the optical output controller 10 and likewise the other parts are shown with “10” being added to the reference numerals of the optical output controller 0 in FIG. 7. Similarly, in the optical output controller 20, “20” is added to the reference numerals of the optical output controller 0 in FIG. 7, as shown by the input portion 21, which is the only different point from the optical output controller 0 in FIG. 7. Since the optical output monitoring signal Vmon which is a feedback signal is common to the optical output controllers 10 and 20, the same reference numeral is used.
Also, in the wired-OR circuit OR, the output terminals of diodes D10 and D20 respectively connected to the output resistors R13 and R23 of the optical output controllers 10 and 20 are mutually connected, so that the optical control signal Vcnt assumes a wired-OR signal of the output Vo10 of the operational amplifier OP10 and the output Vo20 of the operational amplifier OP20 to be provided to the optical variable attenuator OAT.
The operation upon output control (setting) of the optical transmission apparatus 5 will now be described referring to the waveforms of FIGS. 10A and 10B.
Firstly, since −Vref+Vmon>0 before the output control (setting) start time t1 of the monitoring controller 6 as mentioned above, the output voltages Vo10 and Vo20 respectively of the operational amplifiers OP10 and OP20 are amplified up to the negative power supply voltage −V and stabilized as shown in FIG. 10B.
When a common output preset value Vref is provided to the optical output controllers 10 and 20 from the monitoring controller 6, preset value controllers 12 and 22 respectively output inverted preset value signals −Vref10 and −Vref20. The reason why two inverted preset value signals −Vref10 and −Vref20 may be generated is because the D/A conversion and the polarity inverting operation in the preset value controllers 12 and 22 are separately performed. The operational amplifiers OP10 and OP20 control their output voltages Vo10 and Vo20 so that −Vref10+Vmon=0 and −Vref20+Vmon=0 by the respective controls.
In case of a relationship of −Vref10<−Vref20 (reverse relationship for absolute values) as shown in FIG. 10A, it is obtained that (−Vref10+Vmon)<(−Vref20+Vmon). As a result, the relationship between the output voltages Vo10 and the Vo20 is inverted by the inversion amplifying function of the operational amplifier, which leads to Vo10>Vo20 as shown in FIG. 10B.
The optical control signal Vcnt to the optical variable attenuator OAT assumes a wired-OR value by the diodes D10 and D20 of the output voltages Vo10 and Vo20. Therefore, in the above-mentioned example, the optical control signal Vcnt follows the level of the output voltage Vo10 whose voltage value is high, and the optical variable attenuator OAT is controlled to a fixed optical level by the optical output controller 10.
While at this time the optical output controller 10 performs the control so as to obtain −Vref10+Vmon=0, and similarly performs the control so as to obtain −Vref20+Vmon=0, the output voltage Vo20 of the operational amplifier OP20 in the optical output controller 20 after a time t2 of −Vref20≦Vmon is kept being controlled so as to output a negative voltage as shown in FIG. 10B due to −Vref10<−Vref20 as mentioned above, so that the output negative voltage Vo20 assumes the negative power supply voltage −V.
It is to be noted that there has been proposed an automatic optical output control circuit in which a temperature state of a protection LD is detected by a temperature sensor of an automatic optical output control APC circuit, and its output is connected to an analog amplifier through a gate circuit controlled by an LD prebias control signal. When the temperature of an LD varies in a normal APC loop, the amplifier varies the output voltage of the sensor so as to equalize with an output variation width of an analog amplifier compensating a variation of an optical output by controlling an LD DC bias current. The output of the amplifier is connected to the base of a transistor through the transistor and a DC bias current suitable for the temperature state of the LD is flowed even if the output voltage of the amplifier drops at the time of no-signal. Thus, a pulse modulation characteristic of a protection LD can be obtained at the time of an LD switchover without delay of the light emission (see e.g. patent document 1).
Also, in a redundant system which has a 0 system device and a 1 system device for a single or a plurality of devices, and which executes processing e.g. a clock signal by using a clock signal selected and outputted by a selector of each device, there has been proposed a redundant system switchover control method having switchover settlement controllers which mutually transfer state information such as a switchover control state, receive state information of an alarm signal or the like from the device, output a switchover control signal based on state information of its own system and the other system and state information of the alarm signal or the like of the device to be provided to the selector of the device through a switchover control signal line, and the selector performing a system switchover between the 0 system device and the 1 system device corresponding to the switchover control signal of both systems (see e.g. patent document 2).    [Patent document 1] Japanese Patent Application Laid-open No. 60-186138    [Patent document 2] Japanese Patent Application Laid-open No. 6-61985
In the above-mentioned prior art, when e.g. the optical output controller 10 is switched over to the optical output controller 20 due to a unit failure and a unit mounting omission, there has been a problem that the optical output monitoring signal Vmon largely varies as follows:
When such a unit failure or mounting omission occurs in the optical output controller 10 during the optical output control (in the working system) so that the control is disabled, as shown by the operation waveform upon the optical output controller switchover of FIG. 11B, the output voltage Vo10 is dropped from a time t11, followed by the drop of the optical control level Vcnt, and further followed by the drop of the optical output monitoring level Vmon as shown in FIG. 11A. At a time t12 of −Vref20=Vmon, the output voltage Vo20 of the operational amplifier OP20 begins to rise, whereby the optical control level Vcnt is controlled.
However, there has been a problem that the actual optical control signal Vcnt largely varies as shown in FIG. 11B since a time period up to a time t13 of Vo20>Vo10 is required in order to obtain the output voltage Vo20 of the optical output controller 20=optical control signal Vcnt.